1. Field of the Invention
The invention relates to acoustic vibration sensors and acoustic vibration sensing systems in general. The principal application of such sensors and sensing systems is to hydrophones which detect acoustic vibrations in water. Such devices may be used singly or in arrays for both detection and location of sources or scatterers of acoustic vibrations. The sensing achieves sub-angstrom unit sensitivity by use of optical techniques.
2. Description of the Prior Art:
A known acoustic sensor for the hydrophone application consists of a flat coil of single mode optical fibers designed to be supplied with optical energy from a laser. Impinging acoustic energy modulates the optical length of the coil and the change in optical length can be detected by heterodyning the acoustically modulated energy with energy offset by a fixed amount to form an optical heterodyne. The means of offsetting the optical frequency of the laser output is a Bragg modulator, which may be of conventional design. The process produces a pair of optical signals separated by the Bragg modulator frequency, and one of which is acoustically modulated by the vibrations being sensed. A photodetector to which the two optical signals are applied produces an output at the difference frequency. The acoustic signal may be recovered by phase or frequency modulation of the photodetector output.
An objection to known systems is the difficulty of avoiding stray noise pick up in the connections made to the acoustic sensors.